2,2,3,3-tetrafluro-1-propanol (abbreviated as TFP hereinafter) has been used as the solvent in the DVD production process because of its high solubility towards cyanine dyes and its high volatility. TFP is expensive and causes a seriously environmental problem after its emission, it therefore is essential to recover it from the exhausted gases.
There are some existing techniques to remove volatile organic compounds from gases such as condensation, adsorption, catalytic oxidation, and thermal oxidation [1]. But to deal with a ppm concentration level, adsorption onto porous adsorbents is believed to be the most appropriate technique. One of the keys for a successful adsorption process is the adsorbent that generally possesses porous structure and a high adsorptive capacity. Among the adsorbents, activated carbon is the most commonly used due to its high surface area and low cost. Because a huge gas flow rate is needed to treat for the recovery of TFP with a ppm concentration in a DVD plant, the adsorption by activated carbon is therefore chosen in the present invention.
When the adsorption approaches saturation, desorption is needed for the subsequent use. The desorption of activated carbon loaded with different adsorbates including benzene, toluene, ethyl acetate, etc, by supercritical carbon dioxide has been proved to be an effective means. The advantages of using supercritical CO2 as the desorbent over the conventional steam desorption technique include a better subsequent adsorption efficiency due to the same temperature in adsorption and desorption, an absence of the condensed water in activated carbon pores, and a safer operation due to its low desorption temperature. Another important aspect to use supercritical fluid desorption for the recovery of TFP is that there is in general no steam in the display and semiconductor industries.
Because the gas containing TFP is exhausted from a clean room, the adsorption of TFP and desorption of activated carbon should be completed in a limited space. Recently, the high-gravity operation has been proved to be more efficient over the packed-bed tower in different unit operations resulting from a significant increase in mass transfer rate [T. Kelleher and J. R. Fair, “Distillation Studies in a High-Gravity Contactor,” Ind. Eng. Chem. Res., 35, pp. 4646-4655, 1996; M. Keyvani and N. C. Gardner, “Operating Characteristics of Rotating Beds,” Chem. Eng. Prog., 85, pp 48-52, 1989; C. C. Lin and H. S. Liu, “Adsorption in Centrifugal Fields Basic Dye Adsorption by Activated Carbon,” Ind. Eng. Chem. Res., 39, pp. 161-167, 2000; and C. C. Lin; W. T. Liu and C. S. Tan, “Removal of Carbon Dioxide by Absorption in a Rotating Packed Bed,” Ind. Eng. Chem. Res., 42, pp. 2381-2386, 2003]. High gravity (Higee) first proposed by Ranshaw and Mallinson [C. Ramshaw and R. H. Mallinson, Mass Transfer Process, U.S. Pat. No. 4,283,255, 1981] is an operation in a rotating packed bed (abbreviated as RPB hereinafter) in which the centrifugal force replaces the gravitational force. Due to the presence of a centrifugal force, 20 to 100 gravities depending on rotating speed and radius of the rotating bed can be generated. As a result, the equipment size can be significantly reduced. This would definitely be beneficial for the recovery of TFP in a plant with limited space.